1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device using color filters to reproduce colors.
2. Description of the Prior Art
In recent years, numerous display devices have been available in which color filters are used to decompose light from a light source into N colors that are projected onto a screen for reproducing a color image, where N is a positive integer. Normally, N=3, and light is decomposed into red (R), green (G), and blue (B) colors which are projected to reproduce a color image. The simplest example of implementation for achieving this is given below.
FIG. 1 shows an example of a display device, comprising a light source 101, a color wheel 102, a light valve 103, a screen 104, and drive electronics 105. The display device shown in FIG. 1 is assumed to project light decomposed into R, G, and B colors, thus reproducing color images.
The operation of the display device constructed as described above is described by referring to FIG. 1. Seven-bit color image data having a frame rate of 60 Hz and a synchronizing signal are applied to the drive electronics 105. The drive electronics 105 create control signals for the color wheel 102 and for the light valve 103 from the entered color image data and the synchronizing signal. The control signals are fed to the color wheel 102 and to the light valve 103.
The light valve 103 is a device for turning ON or OFF each individual pixel. A digital micromirror device (DMD), a liquid crystal, or the like is used as the light valve 103. Where the DMD is used as the light valve 103, the direction in which light is reflected is controlled for each individual pixel, thus turning ON or OFF the light. Where the light is reflected toward the screen, the device is turned ON. Where the light is reflected toward the outside the screen, the device is turned OFF. This is referred to as control of the reflection.
Where a liquid crystal is used as the light valve 103, the following two types are conceivable. One type controls reflection in the same way as the aforementioned DMD. Another type switches ON and OFF passage of light for each individual pixel. Where the light is transmitted, the device is turned ON. Where the light is not transmitted, the device is turned OFF. The transmitted light is brought to a focus on the screen.
An ultrahigh-pressure mercury lamp is used as the light source 101, for example. Light emitted from this lamp is made to hit a part of the color wheel 102. This color wheel 102 is divided into three segments, for example. These segments are color filters Cr, Cg, and Cb that transmit R, G, and B, respectively. The color wheel 102 makes one revolution in {fraction (1/60)}msec, i.e., about 16.667 msec (3600 rpm). This rotation is synchronized to the frame rate (60 Hz in the above example) of the displayed image.
Where light from the light source 101 shines on the color filter segment Cr on the color wheel 102, the light valve 103 is controlled by color image data about R. An R image is projected onto the screen 104. With other colors, the light from the light source 101 is similarly projected onto the screen 104 via the color filters on the color wheel 102 and via the light valve 103, and images are displayed.
The times for which the light from the light source 101 is made to shine on the segments of the color wheel 102 during one revolution of the color wheel 102 are next described. The light source 101 illuminates parts of the color wheel 102. The produced light spot has some diameter. Where this light spot is at the boundary between two adjacent color filters, two colors across the boundary will be mixed up. That is, one light spot has two colors of light transmitted through the color filters. This cannot be used for image display. Therefore, where the light spot shines on the boundary, it is necessary to turn OFF the light valve.
For the sake of illustration, it is assumed that the light valve must be kept OFF within an angular range of 15° on the color wheel 102. Of course, this angular range may differ, depending on the size of the light spot and on the sizes of the segments forming the color filters.
As can be seen from FIG. 1, the boundaries between the color filters on the color wheel 102 are three boundaries between R, G, and B colors. During one revolution of the color wheel 102, it is necessary to turn OFF the light valve 103 for a time corresponding to an angular range of 15×3=45°. This time is referred to as the ineffective time. The other time is referred to as the effective time.
Since the color wheel 102 makes one revolution in about 16.667 msec, the ineffective time is 45°/360°×16.667≅approximately 2.083 msec. Of the effective time, the time for which the light shines on the color filter Cr is equal to the effective time divided by 3, i.e., about 4.862 msec (({fraction (1/60)}×(1−45°/360°))/3≅16.667−2.083) msec/three colors. Similarly, the time for which the light shines on the color filters Cg and Cb is about 4.862 msec.
A method of reproducing gray levels is now described by taking the case of R as an example. When the light shines on the color filter Cr during the effective time of the color wheel 102, the light valve 103 is controlled according to an R image signal. Where the first gray level is displayed, the light valve 103 is turned ON for about 0.038 msec within the time for which the light shines on the color filter Cr during one revolution of the color wheel 102. The light valve is kept OFF during the remaining time of about 4.824 msec. Where the second gray level is displayed, the light valve 103 is turned ON for twice of the ON time for the first gray level, i.e., 0.076 msec. The light valve is kept OFF during the remaining time of 4.786 msec. Where the third, fourth, . . . , and 127th gray levels are displayed, the light valve is turned ON for 3 times, 4 times, . . . , and 127 times, respectively, of the ON time for the first gray level. The light valve is kept OFF during the remaining times. Thus, there are 128 combinations of ON/OFF times including a fully OFF state.
The human eye does not respond to flickers higher than 60 Hz, which is generally known as the critical flicker frequency. As the ON time prolongs within the 16.667 msec, the human eye feels brighter. As the ON time shortens, the eye feels darker. The human eye perceives 128 ON/OFF time combinations as 128 gray levels. Light is projected onto the screen such that the light valve is turned ON or OFF for each pixel, and an R image that visually has gray levels is reproduced. With respect to each of G and B, 128 gray levels are reproduced in the same way as in the case of R.
Each image of R, G, and B is projected in turn onto the screen for one third of 1 frame time of about 16.667 msec, i.e., about 5.556 msec. As mentioned above, the human eye does snot respond to flickers higher than the critical fusion frequency of 60 Hz and so he or she feels as if three colors were displayed simultaneously. Consequently, a color image is visually reproduced.
In the example given above, gray levels corresponding to 7 bits, i.e., 128 gray levels (27 gray levels), are represented. The light valve 103 is switched ON and OFF at intervals of about 0.038 msec, i.e., the time (about 4.862 msec) for which light is made to shine on the color filter Cr divided by 127 (128−1) that is the number of gray levels excluding the zeroth gray level at which light is not output.
Where it is attempted to display a wider range of gray scale with the above-described structure, e.g., gray levels (28=256 gray levels) corresponding to 8 bits, it is necessary to switch ON and OFF the light valve 103 at intervals within the time for which light is made to shine on the color filter Cr divided by 255, i.e., about 0.019 msec, if the principle described above is applied.
Where the light is turned ON and OFF using the light valve 103 such as a DMD as mentioned above, however, the minimum switching time achievable with the presently available DMD is about 0.030 msec. Therefore, it is impossible to switch the device ON and OFF at intervals of about 0.019 msec as described above.
Where the light is turned ON and OFF using the light valve 103 as consisting of a DMD in an attempt to solve the above-described problem, the minimum switching time is about 0.030 msec as described above. A structure capable of displaying 1024 gray levels (210 gray levels) with this structure is disclosed, for example, in Japanese Unexamined Patent Publication No. 149350/1997.
This disclosed display device is shown in FIG. 2. Note that like components are indicated by like reference numerals in various figures and those components which have been already described in connection with FIG. 1 will not be described below. A color wheel 202 is divided into 6 segments to form color filters Crd, Cgd, and Cbd of lower transmissivity than color filters Cr, Cg, and Cb, in addition to the conventional filters Cr, Cg, and Cb. The transmissivity of the filters Crd, Cgd, and Cbd is one eighth of that of the filters Cr, Cg, and Cb. Thus, gray levels corresponding to the 3 bits, i.e., 23 gray levels (8 gray levels), are added.
The structure shown in FIG. 2 and its operation are now described. Drive electronics 205 receive 10-bit color image data having a frame rate of 60 Hz and a synchronizing signal. The drive electronics 205 create control signals for a color wheel 202 and for a light valve 103 from the input color image data and send these control signals to the wheel and to the light valve.
Of the 6 segments on the color wheel 202, the color filters Cr and Crd transmit R. The color fitters Cg and Cgd transmit G. The color filters Cb and Cbd transmit B. The transmissivity of the filter Crd is one eighth of that of the filter Cr. The transmissivity of the color filter Cgd is one eighth of that of the filter Cg. The transmissivity of the color filter Cbd is one eighth of that of the filter Cb.
The color wheel 202 makes one revolution in {fraction (1/60)}msec≅16.667 msec. This rotation is synchronized to the frame rate of the displayed image. In the structure shown in FIG. 2, there are 6 color filters and so there exist 6 boundaries as can be seen from the figure. In this case, therefore, the ineffective time is about 15°×6/360°×16.667 msec ≅4.167 msec. The effective time is about 16.667 msec−4.167 msec=12.500 msec.
The time for which the light from the light source 101 is made to shine on the color filter Cr of the color wheel 202 during one revolution of the color wheel 202 is one third of the aforementioned effective time (12.500 msec) multiplied by a proportion at which light is made to shine on the color filter Cr, i e., about 12.500 msec/3×127/(127+7)=3.949 msec. The segment of the color filter Cr is determined based on this time. Similarly, the time for which light is made to hit the color filters Cg and Cb is also about 3.949 msec.
The time assigned to illuminate the color filter Crd is one third of the effective time (12.500 msec) multiplied by the proportion at which the filter Crd is illuminated, i.e., about 12.500 msec/3×7/(127+7)=0.218 msec. The segment of the color filter Crd is determined based on this time. Similarly, the time for which the color filters Cgd and Cbd are illuminated is about 0.218 msec.
A method of reproducing gray levels is now described, taking R as an example; The time for which the color filter Cr of the color wheel 202 is illuminated is controlled according to R color image data. Where the first gray scale of the R image signal represented by the filter Cr is displayed, the light valve 103 is turned ON for about 0.031 msec (=3.949 msec/127) of the time for which the filter Cr is illuminated during one revolution of the color wheel 202. The valve 103 is kept OFF during the remaining time.
Where the second gray level represented by the color filter Cr is displayed, the light valve 103 is maintained ON during twice of the ON time for the first gray level represented by the filter Cr, i.e., about 0.062 msec. The valve is kept OFF during the remaining time. Where the third, the fourth, . . . , and the 127th gray levels are displayed, the light valve is turned ON for 3 times, 4 times, . . . , 127 times, respectively, of the ON time for the first gray level. The light valve is kept OFF during the remaining times. Thus, there are 128 combinations of ON/OFF times and thus 128 gray levels can be represented.
A method of displaying 1024 R gray levels using the color filter Crd is now described. Where the first gray level represented by the filter Crd is displayed, the light valve 103 is kept ON for about 0.031 msec (=0.218 msec/7) within the time for which the filter Crd is illuminated during one revolution of the color wheel 202. The valve is kept OFF during the remaining time. Where the second gray-level represented by the filter Crd is displayed, the valve is kept ON for twice of the ON time for the first gray-level represented by the filter Crd, i.e., 0.062 msec. The valve is kept OFF during the remaining time. Where the third, fourth, . . . , and 7th gray levels represented by the filter Crd are displayed, the light valve is kept ON for 3 times, 4 times, . . . , 7 times, respectively, of the ON time for the first gray level represented by the filter Crd. The light valve is kept OFF during the remaining time. Thus, there are 8 combinations of ON/OFF times including a fully OFF state and thus 8 gray levels can be represented.
The transmissivity of the color filter Crd is one eighth of that of the filter Cr. The brightness of the first gray level displayed using only the color filter Crd is one eighth of that of the first gray level displayed using only the filter Cr. That is, using combinations of the color filters Cr and Crd, 128 gray levels (provided by the color filter Cr)×8 gray levels (provided by the color filter Crd)=1024 gray levels (210 gray levels) can be represented.
Accordingly, of color image data (R image data in this example) quantized with 10 bits (210), the upper-order 7 bits are expressed using the color filter Cr, while the lower-order 3 bits are expressed using the color filter Crd. In this way, 1024 gray levels can be reproduced.
With respect to G and B, the upper-order 7 bits are expressed using the color filters Cg and Cb. The lower-order 3 bits are represented using the color filters Cgd and Cbd. In this manner, 1024 gray levels can be reproduced. Images of R, G, and B are projected onto the screen 104 by this gray scale control. A color image is perceived by the human visual characteristics.
Where 1024 gray levels are expressed using the structure and procedure described above, the light-transmitting region of the color wheel 202 is divided into 6 segments corresponding to the different colors and different gray levels. Therefore, there are 6 boundaries between the color filters. The ineffective time due to the boundaries is doubled compared with the case in which there are only three boundaries between color filters. Finally, the brightness of the image projected onto the screen is decreased by about 14%.
In addition to this decrease in the brightness, the presence of the color filters Crd, Cgd, and Cbd having a transmissivity that is only one eighth of that of the color filters Cr, Cg, and Cb lowers the brightness.